Well, what settings for DPI/ bleed and what have you do you use in the PDF dialog? Sounds like a simple rounding/ quantization issue with classic point size vs. computerized point size and all calculations based thereon...
A way to see what's going on is this:
EXACTLY how many pixels do you get in 4.625 inches at 300 ppi? Do the math (i.e., multiply those numbers). You'll see it's not an integral number of pixels.
A computer image MUST have an integral number of pixels in it. Since Photoshop can't make up the half pixel for you, it rounds down.
Move up to 400 ppi and you'll get an integral number and no rounding.